- Extract core StringPrototypeRepeat code into a TFS builtin (StringRepeat)
  - Assumes arguments are a string and smi (no range checks)
- Add StringPrototypePadStart and StringPrototypePadEnd TFJ builtins
  - Added StringPadAssembler to ensure common behavior
- Removed functionality from string.js

A quick benchmark shows significant performance gains for unoptimized
code (2.1x to 2.46x) and optimized code (1.03x - 1.56x).

https: //github.com/peterwmwong/v8-perf/blob/master/string-pad/README.md
Bug: v8:5049
Change-Id: I6e4fe99fb62a3edb3d6906fd4f78b3576b5b0d13
Reviewed-on: https://chromium-review.googlesource.com/720067
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48595}